For typical industrial applications, high purity oxygen is typically produced and supplied at a purity of about 99.5% or higher. This high purity oxygen may be produced on-site, provided to the site by means of a pipeline network, or brought into the site in liquid form, and stored in tanks. For pipeline applications, typically, the pipeline operating pressure is 40 bar or higher. However, there are numerous applications, such as combustion, that require neither this purity level, nor a delivery pressure this high.
Applications, such as glassmaking, steelmaking and gasification for energy production can use low purity oxygen. Typically, for glassmaking or steelmaking applications, the acceptable oxygen purity level may be as low as about 90%. Typically, for gasification applications, the required oxygen purity level may be as low as about 95%, or even about 85%.
The power required to compress the gaseous oxygen to such high pressure contributes significantly to the production and supply cost of the product oxygen. This cost of compression can account for more than about 50% of the energy required to perform the separation of oxygen from air. As applications develop and are identified that require relatively impure, low pressure oxygen, a solution is required that will allow such a product to be provided to the customer in a cost-effective way.
The person skilled in the art would recognize that there are currently five basic solutions to the above problem.
The first solution would be to use a VSA or PSA based system. Vacuum swing adsorption (VSA) or pressure swing adsorption (PSA) processes use a non-cryogenic technology based on nitrogen adsorption through a molecular sieve. These types of units produce low purity oxygen, typically between about 90% and about 95% purity. Typically, the VSA produces oxygen at around 1.03 bar, and the PSA produces oxygen at between about 2 bar and 4 bar. These technologies tend to be moderately cost effective, however, the reliability of this technology requires that expensive liquid backup systems be installed. Other non-cryogenic adsorption processes, such as temperature swing adsorption (TSA), temperature-pressure swing adsorption (TPSA) process may also be used, but suffer from similar disadvantages. The adsorption solution is described in several technical papers, such as U.S. Pat. Nos. 5,114,440, 5,679,134, and 6,332,915.
The second solution would be to use a small, standardized, pre-designed and modularized cryogenic air separation unit. These units produce moderately low purity oxygen, typically between about 95% and about 98.5%. This technology is more reliable than the adsorption based technologies, however, a liquid backup system would typically still be required. This cryogenic process is usually the basic double column process and is widely used in the air separation industry.
The third solution would be to co-produce low purity and low pressure oxygen from a liquid production plant and utilize the existing storage facilities as the backup source
The fourth solution would be to install an air separation plant that would provide oxygen at two purity levels. Typically, if all the oxygen is removed at a lower purity level, the energy requirement of the plant may be reduced on the order of about 10%. A basic cryogenic air separation plant is actually more efficient if, for example, one half of the oxygen is extracted at a high purity level, and the other half of the oxygen is extracted at a lower purity level, and if a traditional power usage is assigned to the high purity portion then the resulting power usage for low purity oxygen portion can be reduced on the order of about 20%.
This sort of bi-purity arrangement is advantageous, for example, for iron metallurgy applications. The blast furnace may require a lower purity oxygen, while the steelworks may require a higher purity oxygen. While this type of solution works well in theory, practical considerations, such as customer demand changes, fluctuations in the different loads, etc. make this solution marginal at best.
The fifth, and final, solution would be to utilize an oxygen pipeline. This solution is only available should the consumer be in close proximity to an existing pipeline. Typically, these pipelines operate at pressures of about 40 bar or higher. Such a solution would require taking high value added, high purity and high pressure oxygen and reducing the pressure to provide low pressure oxygen at a much higher purity than required by the customer. This solution is not efficient since high purity and high pressure oxygen is used to supply a demand for low purity and low pressure. Its first investment cost is low and can only be used for short term needs but not for long term operations.
For the foregoing reasons, a need exists within the industry for a system that will provide low purity and low pressure oxygen to a customer at an economically attractive price.